External defibrillators frequently include a pair of “hands-free” disposable electrodes, which are essentially flexible pads that are adhered to the skin of a patient having a cardiac event (i.e., used transcutaneously). By “hands-free,” we mean electrodes of the type that are adhered to a patient, rather than paddles that are held by a rescuer during defibrillation. Hands-free disposable electrodes typically include a metal layer, formed from a thin sheet of metal or a conductive ink printed on a substrate, and a liquid or solid electrically conductive gel covering the metal layer so that electrical current passes through the gel to the patient's body. The metal layer may be, for example, tin or silver-silver chloride.
It is important that the material used in the metal layer depolarize quickly (within seconds) after a defibrillating pulse (“shock”) is delivered to a patient. Otherwise, the electrode is not capable of sensing a signal that will allow the defibrillator to generate a clear ECG and determine whether another shock should be delivered within a short period of time.